1. Field of the Invention
The invention relates to a method and apparatus for regulating throughput in a beltpress machine.
2. Related Art
Beltpress machines are used in a variety of applications, including processing sludge and separating juice and pulp from fruit. The machine is basically operated on the principle of pressing feed material such as sludge or fruit between two moving belts. The pressing is accomplished by moving the belts over a series of rollers and sequentially narrowing the spacing between the belts so as to press the feed material therebetween. During the pressing operation, water or juice is forced outward through the belts or beyond their edges and is collected by suitable means. At the end of the pressing operation, the partially dewatered feed material is expelled from the belts.
Each of the pair of belts used for the pressing operation is moved over separate drive and idler rollers in an endless loop. Typically, feed material is deposited on one of the belts at a section where the belt is horizontally aligned and the deposited feed material is moved along by the one belt and brought into contact with the opposing belt. In one known beltpress, described in U.S. Pat. No. 3,984,329 to Wenzel, the feed material is brought into contact with the opposing belt at a section where both belts are sharply vertically inclined. In this beltpress, the feed material is initially squeezed between the belts as it slides downward between the vertically inclined section of the belt in a chute-like manner. In another known beltpress, the feed material is brought into contact with the opposing belt at a section of the beltpress where two opposing belts are approximately horizontal.
Differences in the rate of travel of each belt can arise when feed material of varying thickness is initially pressed. This differential rate of travel of the two belts leads to undesirable consequences including one of the belts folding on itself, often necessitating shutdown of the beltpress operation to unfold the belt. U.S. Pat. No. 4,836,100 to Johnson et al. employs a squeeze plate operatively connected to a rotating pipe mounting on the frame of the beltpress. The vertical spacing between the squeeze plate and the pipe varies during rotation of the pipe so that the squeezing action of the plate can be selectively varied.
Sludge generally can be grouped into three categories: biological, mineral and industrial. Water is normally held in these materials at three different levels, each requiring a different method of removal. Free water, which is sludge retained water, can be easily removed by sedimentation or normal drainage. Intercellular water is held to the sludge particles by specific chemical bonding, requiring the use of the polymer to break the bond. Intracellular water is contained inside the individual cells of the sludge, and can be removed only by using costly processes to break the cell wall.
It is known to mix a polymer into a sludge as a flocculate. Polymers have played an increasingly important role in waste water treatment and today they are one of the most important factors in sludge dewatering. When mixed with the sludge, polymers combine with the charged particles, neutralizing the charge which permits the agglomeration of the particles. For example, a positively charged polymer is introduced into negatively charged municipal sludge. This causes the polymer and the sludge to coagulate resulting in the separation of water and other liquids. This process, called flocculation, releases the free and intercellular water which results in a sludge that can be mechanically dewatered.
It is also known to use a combination of gravity and mechanical pressure to process sludge. By evenly distributing the flocculated sludge on a porous belt, the free water, which is often over 50% of the sludge volume is removed by gravity. In both a low and high pressure compression stage, the sludge is "sandwiched" between two belts and routed over various size rollers to create a pressure/shear effect, which removes additional water and produces a cake ready for disposal.
Cost effective implementation of such systems requires the efficient use of such polymers. Conventional systems have relied on manual manipulation to regulate mixing of the polymer with the sludge. For example, an operator first visually inspects the output cake. Based on manually observed cake thickness, rather than on automatic or mechanically assisted measurements, the operator adjusts an insert feed plate to obtain a desired cake thickness output. Often the operator fails to adjust the feed plate, resulting in inefficient system operation, excess water in the output cake product and overconsumption of polymer.
Other conventional systems have employed a Streaming Current Detector to measure charge in a sludge polymer mix and adjust the polymer flow. Another conventional system has also adjusted the mixing energy based on the charge measurements of a Streaming Current Detector. Other approaches to controlling polymer flow have also been employed. In another system, the rheology of a sample of flocculated material is determined, compared to a baseline and polymer flow is adjusted accordingly. Another system measures refracted light in a gravity portion of the system and the polymer flow is increased or decreased according to the measurement.
The conventional systems discussed above fail to recognize that one factor in determining how the polymer is to be mixed with the sludge to obtain an output rate of dry solids is the rate at which the sludge is introduced into the beltpress. Thus, such conventional systems are limited in their ability to regulate the dewatering process because they do not regulate sludge flow based on characteristics, such as thickness, of the cake produced for disposal.